Prompt gamma correction for non-standard isotopes in a PET scanner

ABSTRACT

A method for correcting PET emission data for prompt gamma emission background components present in non-pure positron-emitting isotopes uses a two component fit of modeled scatter and modeled prompt gamma emission in the area of scatter tails in a normalized emission sinogram. The method allows a PET scan using non-standard PET isotopes to be quantitative and thus more clinically useful.

CROSS-REFERENCE TO RELATED APPLICATION AND CLAIM OF PRIORITY

This application is a non-provisional under 35 U.S.C. §119(e) and claimspriority of Provisional Application Ser. No. 60/914,419 filed Apr. 27,2007.

TECHNICAL FIELD

The current invention is in the field of nuclear medical imaging.Particularly, the invention relates to techniques for correction ofimage acquisition data in Positron Emission Tomography (PET) to enablePET scans to be quantitative.

BACKGROUND OF THE INVENTION

Medical imaging is one of the most useful diagnostic tools available inmodern medicine. Medical imaging allows medical personnel tonon-intrusively look into a living body in order to detect and assessmany types of injuries, diseases, conditions, etc. Medical imagingallows doctors and technicians to more easily and correctly make adiagnosis, decide on a treatment, prescribe medication, perform surgeryor other treatments, etc.

There are medical imaging processes of many types and for many differentpurposes, situations, or uses. They commonly share the ability to createan image of a bodily region of a patient, and can do so non-invasively.Examples of some common medical imaging types are nuclear medical (NM)imaging such as positron emission tomography (PET) and single photonemission computed tomography (SPECT). Using these or other imaging typesand associated machines, an image or series of images may be captured.Other devices may then be used to process the image in some fashion.Finally, a doctor or technician may read the image in order to provide adiagnosis.

A PET camera works by detecting pairs of gamma ray photons in timecoincidence. The two photons arise from the annihilation of a positronand electron in the patient's body. The positrons are emitted from aradioactive isotope that has been used to label a biologically importantmolecule like glucose (a radiopharmaceutical). Hundreds of millions suchdecays occur per second in a typical clinical scan. Because the twophotons arising from each annihilation travel in opposite directions,the rate of detection of such coincident pairs is proportional to theamount of emission activity, and hence glucose, along the lineconnecting the two detectors. In a PET camera the detectors aretypically arranged in rings around the patient. By consideringcoincidences between all appropriate pairs of these detectors, a set ofprojection views can be formed each element of which represents a lineintegral, or sum, of the emission activity in the patient's body along awell defined path. These projections are typically organized into a datastructure called a sinogram, which contains a set of plane parallelprojections at uniform angular intervals around the patient. A threedimensional image of the radiopharmaceutical's distribution in the bodycan then be reconstructed from these data.

Most PET scans are performed using pure positron emitters, and can bemade quantitative by performing normalization, attenuation correctionand scatter correction processes on the acquired image data. Singlegamma background can be removed from the image data acquisition of suchpure positron emitter isotopes through the use of time-coincidencedetection. However, there are isotopes that decay through the emissionof a positron while the nucleus remains in an excited angular momentumstate, leading to a prompt gamma emission (e.g., within about 0.1 nsecof the annihilation gamma pair in liquids or solids and 10-100 nsec inatmospheric air). This solitary gamma plus the two annihilation gammaphotons (E=511 keV) derived from a positron-electron annihilation yieldsa triple of coincident gammas with known energies. Thus the net decaysignature is a 511 keV gamma pair traveling in opposite directions and asolitary gamma with a non-correlated emission direction and distinctenergy. When a non-standard PET isotope is used, therefore, the promptgamma background component additionally must be compensated for in theacquired data.

Prior efforts have attempted to compensate for the prompt gammacomponent by using a flat background or a modeled prompt gammadistribution in the non-scatter tails of the sinogram representation ofthe acquired projection data. Such methods however have proven to beinaccurate.

SUMMARY OF THE INVENTION

In accordance with the present invention a method of correcting acquiredPET projection data is provided that achieves an accurate quantificationof a PET scan obtained using non-standard PET isotopes. In particular, atwo-component fit of modeled scatter and modeled prompt gamma emissionis carried out in the area of scatter tails in a normalized emissionsinogram. The scatter tail area provides better statistics for the fit,thus resulting in a better match to the data and a more accuratequantification of the PET emission data.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in greater detail in the followingby way of example only and with reference to the attached drawings, inwhich:

FIG. 1 is a table illustrating steps of a method in accordance with anembodiment of the current invention;

FIGS. 2A-2E depict images of various PET image data distributions atdifferent steps of the method;

FIGS. 3A-3E are graphs of various model factor profiles used in thecompensation method in accordance with an embodiment of the invention;and

FIG. 4 is a graph of distributions of emission, least-squares-fit, andscatter tail data over a selected displacement angle range.

DETAILED DESCRIPTION OF THE INVENTION

As required, disclosures herein provide a detailed embodiment of thepresent invention; however, the disclosed embodiment is merely exemplaryof the invention that may be embodied in various and alternative forms.Therefore, there is no intent that specific structural and functionaldetails should be limiting, but rather the intention is that theyprovide a basis for the claims and as a representative basis forteaching one skilled in the art to variously employ the presentinvention.

FIG. 1 is a table of a method according to the present invention. Atstep 110, nuclear medical image projection data is obtained from a PETscanner. The data is conventionally sent to a processor from the PETscanner in the DICOM (Digital Imaging and Communications in Medicine)standard. The data may be converted from the DICOM standard to anotherstandard, such as Interfile, if needed. The processor generates from theinputted DICOM data from the PET scanner an emission sinogram (em.s),attenuation factors (attn.a), and normalization factors (norm.n),together with associated headers (.hdr). In one embodiment, theattenuation data is obtained from a CT scanner used to obtainattenuation data, but the attenuation data also could come from the PETscan itself or any other imaging modality that may be merged with thePET scanner, e.g. MR/PET, SPECT/PET, CT/PET etc.

At step 120, the emission sinogram em.s (i.e., without scatter orattenuation correction) is normalized using the normalization factorsnorm.n, to obtain a normalized emission sinogram (emc.s). See FIGS. 2A,3A. At step 130, the contribution of scatter to the emission data isestimated using the attenuation factors attn.a in a scatter simulationon the normalized emission sinogram emc.s, to obtain a scatter sinogram(scat.s). See FIGS. 2C, 3B, 3C, 3D.

At step 140, the sinogram tails are found using the attenuation andscatter sinograms, to obtain a tails sinogram tails.s. At step 150, Arandoms sinogram randoms.s is generated from the singles rates in theemission sinogram header (em.hdr). See FIGS. 2B, 3E. This randomssinogram serves as the model of the prompt gamma component of theemission data. However, it is to be noted that the prompt gamma modelmay be constructed from other suitable data that accurately representsthe contribution of prompt gamma emission to the collected PET data. Forexample, the prompt gamma background may be estimated using smoothedrandoms (e.g. data from the scanner), or using a computer simulationbased on scanner data and isotope.

Next, at step 160 the background radiation is removed from thenormalized emission sinogram. This background is modeled as a linearcombination of the randoms (random.s) and the scatter (scat.s). Thesetwo components are used in a least-squares fit with the sinogram tails(tails.s), to obtain a “clean” sinogram (clean.s) in accordance with thefollowing equation, wherein k and j are appropriate compensationcoefficients:clean=emc(t)−k*randoms(t)−j*scat(t)  (1)

At step 170, the clean sinogram (with background removed) is correctedfor attenuation using the attn.a factors (see μ-map, FIG. 2E), to obtaina fully corrected final sinogram (final.s). At step 180, an image(image.v) is reconstructed from the fully corrected final sinogramfinales, using a known reconstruction algorithm such as Filtered BackProjection (FBP), as shown in FIG. 2D.

The performance of the least-squares fit of the randoms and scattermodels with respect to the sinogram scatter tails is shown in FIG. 4 fora displacement angle range from 0 to 15 degrees, wherein other angleranges over a 0 to 180 range will show similar characteristics. Thedot-dash line illustrates the estimation of prompt gamma backgroundcontained in the emission data in accordance with the invention, and thethick line illustrates the estimation of background taking into accountthe prompt gamma background. Also shown as a dashed line is how thebackground would be estimated (i.e., due to scatter alone) withoutknowledge of the prompt gamma component. As shown, the correction ofemission PET data using the present invention provides a significantlymore accurate background estimation including prompt gamma backgroundthan the prior art.

The invention having been thus described, it will be apparent to thoseskilled in the art that the same may be varied in many ways withoutdeparting from the spirit and scope of the invention. Any and all suchvariations are intended to be included within the scope of the followingclaims.

1. A method for correcting PET emission data for prompt gamma emissionbackground, comprising the steps of: obtaining PET emission dataoutputted from a PET scanner; estimating scatter data included in saidoutputted PET emission data; calculating emission tail distribution dataof said outputted PET emission data; estimating prompt gamma backgroundin said outputted PET emission data; fitting said estimated prompt gammabackground and estimated scatter data in said tail distribution data toobtain estimated background data; and subtracting said estimatedbackground data from said outputted PET emission data to obtaincorrected PET emission data.
 2. A method for correcting PET emissiondata as set forth in claim 1, further comprising the step of normalizingthe outputted PET emission data prior to estimating scatter data.
 3. Amethod for correcting PET emission data as set forth in claim 1, furthercomprising the step of converting said outputted PET emission data fromsaid PET scanner into an emission sinogram.
 4. A method for correctingPET emission data as set forth in claim 3, wherein the step ofestimating scatter comprises performing a scatter simulation on saidemission sinogram to obtain a scatter sinogram.
 5. A method forcorrecting PET emission data as set forth in claim 4, further comprisingusing attenuation data in said step of estimating scatter.
 6. A methodfor correcting PET emission data as set forth in claim 4, wherein saidstep of calculating emission tail distribution data comprises using saidscatter sinogram and attenuation data.
 7. A method for correcting PETemission data as set forth in claim 1, wherein the step of fittingcomprises performing a least-squares fit (LSF).
 8. A method forcorrecting PET emission data as set forth in claim 1, further comprisingthe step of correcting said corrected PET emission data for attenuationusing attenuation data obtained from said PET scanner, to obtain finalPET emission data.
 9. A method for correcting PET emission data as setforth in claim 8, further comprising the step of reconstructing a PETimage from said final PET emission data.
 10. A method for correcting PETemission data as set forth in claim 9, wherein the step ofreconstructing comprises performing a filtered back-projection on saidfinal PET emission data.
 11. A method for correcting PET emission dataas set forth in claim 1, wherein the step of calculating emission taildistribution data comprises using said estimated scatter data tocalculate said emission tail distribution data.
 12. A method forcorrecting PET emission data as set forth in claim 1, wherein the stepof calculating emission tail distribution data comprises usingattenuation data to calculate said emission tail distribution data. 13.A method for correcting PET emission data as set forth in claim 1,wherein the step of estimating prompt gamma background comprisescalculating randoms data from singles rate data included in saidoutputted PET emission data and using said calculated randoms data tomodel prompt gamma background.
 14. A method for correcting PET emissiondata as set forth in claim 1, wherein the step of estimating promptgamma background comprises performing a prompt gamma backgroundsimulation and using said simulation to model prompt gamma background.15. A method for correcting PET emission data as set forth in claim 1,wherein the step of estimating prompt gamma background comprises usingsingles data to model prompt gamma background.